Technical Field
The present invention generally relates to novel materials exhibiting extremely durable intercalation of lithium, methods for their manufacturing, and methods of their use, for example for energy storage applications. The novel materials comprise a porous scaffold, for example a carbon exhibiting a pore volume comprising micropores, mesopores, and/or macropores, wherein said volume is impregnated with silicon, in some embodiments, the impregnated silicon is nano-sized and/or nano-featured. The silicon-impregnated porous scaffold can be further coated to reduce any remaining surface area, for example, coated with carbon or conductive polymer. Such silicon-impregnated carbon materials and carbon- or conductive polymer-coated silicon-impregnated carbon materials exhibit remarkable durability with respect to their intercalation of lithium. Accordingly, the materials disclosed have utility either alone or in combination with other materials, for example, combined with carbon particles, binders, or other components to provide a composition of matter for energy storage applications. Said energy storage applications include employing the materials herein as electrode materials, particularly anode materials, for lithium ion batteries and related energy storage device employing lithium or lithium ions, for instance lithium air batteries. In certain embodiments, the materials disclosed herein have utility as anode materials for energy storage devices such as lithium ion batteries and related energy storage device employing lithium or lithium ions. Thus, the present invention also relates to compositions and devices containing such materials and methods related to the same.
Description of the Related Art
Lithium-based electrical storage devices have potential to replace devices currently used in any number of applications. For example, current lead acid automobile batteries are not adequate for next generation all-electric and hybrid electric vehicles due to irreversible, stable sulfate formations during discharge. Lithium ion batteries are a viable alternative to the lead-based systems currently used due to their capacity, and other considerations. Carbon is one of the primary materials used in both lithium secondary batteries and hybrid lithium-ion capacitors (LIC). The carbon anode typically stores lithium in between layered graphite sheets through a mechanism called intercalation. Traditional lithium ion batteries are comprised of a graphitic carbon anode and a metal oxide cathode; however such graphitic anodes typically suffer from low power performance and limited capacity.
Silicon, tin, and other lithium alloying electrochemical modifiers have also been proposed based on their ability to store very large amounts of lithium per unit weight. However, these materials are fundamentally limited by the substantial swelling that occurs when they are fully intercalated with lithium. This swelling and shrinkage when the lithium is removed results in an electrode that has limited cycle life and low power. The solution thus far has been to use very small amounts of alloying electrochemical modifier in a largely carbon electrode, but this approach does not impart the desired increase in lithium capacity. Finding a way to increase the alloying electrochemical modifier content in an anode composition while maintaining cycle stability is desired to increase capacity. A number of approaches have been utilized involving nano-structured alloying electrochemical modifier, blends of carbon with alloying electrochemical modifier, or deposition of alloying electrochemical modifier onto carbon using vacuum or high temperature. However none of these processes has proven to combine a scalable process that results in the desired properties.
The aforementioned swelling associated with certain materials, for example silicon materials, upon their intercalation of lithium is a critical factor in the stability, i.e., cycle life of said materials with regards to their application for energy storage and distribution, for example, use in rechargeable batteries. Over many cycles, the capacity of said materials is susceptible to fading. This capacity fade may be precipitated by a variety of different mechanisms, and one of the critical mechanisms thus described related to the formation of a solid-electrolyte interphase (SEI) in the negative electrode, with competes with reversible lithium intercalation. It is known in the art that SEI is a critical component of capacity fade as the canonical degradation mechanism, which can be modeled over long times, based on accelerated aging for short times and elevated temperatures.
It is described in the art that SEI layer plays an important role in the safety, power capability, and cyclic life of Li-ion batteries. It is also described that formation of a chemically and mechanically stable SEI layer is important for improving the cycle life of lithium-ion batteries. The SEI layer on silicon in an anode forms due to reduction of organic solvents and anions at the electrode surface during charging and discharging cycles of batteries, with a substantial degree of the formation happening during the first cycle. Furthermore, certain electrolyte additives, such as vinylene carbonate, propylene carbonate, lithium difluoro-oxalatoborate, and fluoro-ethylene carbonate, and other species known in the art, and combinations thereof, can dramatically improve the cyclic efficiency of silicon-based anodes. SEI layers can comprise fluorinated carbon and silicon species, besides the usual Li2CO3, alkyl Li carbonates (ROCO2Li) (lithium carboxylate), LiF, ROLi (lithium alkoxide), and polyethylene oxides that are found on graphite electrodes. SEI formation on the negative electrode is an irreversible reaction that consumes cyclable Li-ions from the positive electrode leading to most of the capacity loss observed in the first lithiation/delithiation cycle of secondary lithium-ion batteries. Besides capacity loss in the first cycle, continuous formation of this layer also increases resistance to Li-ion diffusion (i.e., internal impedance of a battery).
The repeated expansion and contraction of silicon-based anode materials leads to instability of SEI, for example, cracking and reformation, concomitantly contributing to the capacity fade of the anode. To this end, the art describes a variety of different silicon size and geometries that are preferred in order to avert fracture and mitigate the propensity for chemical and mechanical degradation that can occur upon cycling in a lithium-ion battery. To this end, the art (RSC Advances, 2013, 3, 7398, “Critical silicon-anode size for averting lithiation-induced mechanical failure of lithium-ion batteries, Ma et al.) describes 90 nm as a critical size for nanoparticles, 70 nm for nanowires, and 33 nm for nanofilms, below these dimensions (for their respective geometries) the silicon nanostructures remain undamaged upon lithiation. Another report in the art (DOI: 10.1002/anie.200906287, “A Critical Size of Silicon Nano-Anodes for Lithium Rechargeable Batteries,” Angewandte Chemie, Vol 49, Iss. 12, pp 2146-2149, 2010, Kim et al.) describes for well-dispersed silicon nanocrystals that an approximate size of 10 nm showed higher capacity retention compared to 5 nm or 20 nm sizes.
Additionally, nano features are important to both prevent pulverization of silicon during expansion and contraction as well as retain an amorphous structure throughout cycling. Pulverization is identified as a mechanical failure of silicon due to extreme strain gradients through the bulk structure. As silicon is lithiated, it will expand in volume (upwards to 300%). Lithium ions move very slowly through solid silicon. During lithium insertion, a silicon particle may hold large amounts of lithium near the surface and none in the center of the particle. The concentration gradient creates a non-uniform expansion through the cross section. The extreme surface volume expansion will cause the silicon particle to tear apart away from the inside, cracking and fracturing. Once silicon has pulverized the cell will fail, as there is no known method to salvage the performance.
Accordingly, for energy storage applications, the preferred silicon size is less than 1 micron, preferable less than 800 nm, preferably less than 300 nm, preferably less than 150 nm, preferably less than 100 nm, preferably less than 90 nm, preferably less than 70 nm, preferably less than 50 nm, preferably less than 33 nm, preferably less than 20 nm. In certain instances, the preferred silicon size is between 5 and 20 nm. In specific instances, the preferred silicon size is less than 90 nm for a nanoparticle. In specific instances, the preferred silicon size is less than 70 nm for a nanowire. In specific instances, the preferred silicon size is less than 33 nm for a nanofilm.
A silicon particle of the dimensions described above is generally referred to as a nano-sized silicon particle. The particle size is typically described as the Dv,50 or silicon particle size at 50% of the volume distribution, as measured by various methods known in the art, for instance by laser diffraction particle sizing techniques.
Alternatively, or in addition the silicon exhibiting a primary particle size in the ranges described above, the silicon particle can also exhibit nano features. The silicon nano-features preferably comprise a nano feature size less than 1 micron, preferably less than 300 nm, preferably less than 150 nm, preferably less than 100 um, preferably less than 50 nm, preferably less than 30 nm, preferably less than 15 nm. A silicon particle with the features described above is generally referred to as a silicon particle with nano-sized features. The nano-sized features can be discerned by various methods known in the art, for instance by scanning electron microscopy.
Current technologies for achieving nano sized silicons are expensive and difficult to scale. For instance, the first commonly recognized successful production of Si nanoclusters was reported by Heath and co-workers (Science 1992, 258, 1131; P. E. Batson, J. R. Heath, Phys. Rev. Lett. 1993, 71, 911), and involved the reduction of SiCl4 at high temperature and high pressure in a bomb fitted into a heating mantle. In another example, a process utilized SiCl4 reduction at room temperature under an inert atmosphere, however the product obtained at room temperature was not fully crystallized, requiring further high temperature annealing. Similar solution syntheses have been reported at low or high temperature after reducing silicon salts with LiAlH4 or alkyl silanes, however, all such methods produce a broad particle size distribution or involve aggregation of the nanoparticles. Furthermore, these approaches are not suitable for enabling commercial utility; the scalability and material yields are insufficient to allow for their use in anode production for lithium secondary batteries.
Therefore, the need remains in the art for easily scalable, inexpensive, and improved processes for producing porous silicon materials comprising nano-sized particles and/or exhibiting nano-features that, upon combination with a suitable hard carbon material, can generate the desired electrochemical properties. The current invention meets this need, and provides further related advantages.